1. Field of the Invention
The present invention relates to active pixel sensors and in particular to an active pixel sensor which provides improved red absorption while providing improved latch-up and noise reduction.
2. Description of the Related Art
A conventional Active Pixel Sensor (APS) cell consists of an np diode in the p-type substrate, an NFET transfer device, a source-follower amplifying transistor, a pre-charge transistor, and a bit switch transistor. Electron-hole pairs are produced in the diode by impinging photons. The electrons are collected in the pre-charged n region of the diode, and eventually transferred to the source-follower gate for amplification. A p-type pinning layer is often included at the surface of the diode.
FIG. 1a illustrates a schematic cross-section of the photodiode and the transfer gate. FIG. 1b illustrates the schematic of the circuit embodied in each Active Pixel Sensor device 10. The number of electrons collected determines the level of gate drive on the source-follower transistor 12. The source potential is transferred down the column through the bit switch 14, where it can be processed appropriately for video imaging.
Photons are absorbed by the silicon in the pre-charged n region of the diode at various depths, depending on the photon energy. Long wavelength photons have an absorption depth of 5 or more micrometers. Electron-hole pairs are created when the photons are absorbed, and provided that this creation occurs within a diffusion length of the depletion region, they will be collected by the photodiode and contribute to the optical signal.
Heavily-doped silicon has a short diffusion length, so a deep region of lightly-doped material is preferable to enable the collection of the electron-hole pairs generated by long wavelength red photons. However, high performance CMOS technologies of the type preferred for the logic areas outside of the photodiode use a shallow lightly-doped region for latch-up immunity and to provide a quiet substrate. This causes difficulties when integrating the manufacturing steps needed for the APS device with the manufacturing steps needed for the high performance CMOS logic areas.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an active pixel cell device which optimizes red response and latch-up simultaneously.
It is another object of the present invention to provide an active pixel sensor which allows easy integration with existing embedded microprocessor and analog designs necessary for the high level of integration required by products using APS technology.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.